BONE PATHOLOGYDariusz Borys MDdborys@lumc.eduwww.pedorthpath.com

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OBJECTIVES

Identify main functions of bone tissueIdentify the major parts of a long bonesDescribe the cells found in bone tissueDescribe the components of an osteonCompare and contrast intramembranous and endochondral ossification.

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BONE FUNCTIONSupportProtection (protect internal organs)Movement (provide leverage system for skeletal muscles, tendons, ligaments and joints)Mineral homeostasis (bones act as reserves of minerals important for the body like calcium or phosphorus)Hematopoiesis: blood cell formationStorage of adipose tissue: yellow marrow

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SHAPE OF BONESLong bones (e.g., humerus, femur)Short bones (e.g., carpals, tarsals, patella)Flat bones (e.g., parietal bone, scapula, sternum)Irregular bones (e.g., vertebrae, hip bones)

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BONE ANATOMYDiaphysis: long shaft of boneEpiphysis: ends of boneEpiphyseal plate: growth plateMetaphysis: b/w epiphysis and diaphysisArticular cartilage: covers epiphysisPeriosteum: bone covering (pain sensitive)Sharpeys fibers: periosteum attaches to underlying boneMedullary cavity: Hollow chamber in bone- red marrow produces blood cells- yellow marrow is adiposeEndosteum: thin layer lining the medullary cavity

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BLOOD AND NERVE SUPPLY OF BONEBone is supplied with blood by:Periosteal arteries accompanied by nerves supply the periosteum and compact boneEpiphyseal veins carry blood away from long bonesNerves accompany the blood vessels that supply bonesThe periosteum is rich in sensory nerves sensitive to tearing or tension

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LONG BONESCompact Bone dense outer layerSpongy Bone (cancellous bone) honeycomb of trabeculae (needle-like or flat pieces) filled with bone marrow

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COMPACT BONE: (OSTEON) EXTERNAL LAYER

- called lamellar bone (groups of elongated tubules called lamella)- majority of all long bones- protection and strength (wt. bearing)- concentric ring structure- blood vessels and nerves penetrate periosteum through horizontal openings called perforating (Volkmanns) canals.

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COMPACT BONE: (OSTEON) EXTERNAL LAYERCentral (Haversian) canals run longitudinally. Blood vessels and nerves.

- around canals are concentric lamella- osteocytes occupy lacunae which are between the lamella- radiating from the lacunea are channels called canaliculi (finger like processes of osteocytes)

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COMPACT BONELacunae are connected to one another by canaliculiOsteon contains: - central canal- surrounding lamellae- lacunae- osteocytes- canaliculi

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SPONGY BONE (CANCELLOUS BONE): INTERNAL LAYER

- trabecular bone tissue (haphazard arrangement).- filled with red and yellow bone marrow- osteocytes get nutrients directly from circulating blood.- short, flat and irregular bone is made up of mostly spongy bone

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HISTOLOGY OF BONEHistology of bone tissue

Cells are surrounded by matrix.- 25% water- 25% protein- 50% mineral salts

4 cell types make up osseous tissueOsteoprogenitor cellsOsteoblastsOsteocytesOsteoclasts

Cells of Bone Tissue

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Osteoprogenitor cells:

- derived from mesenchyme- all connective tissue is derived- unspecialized stem cells- undergo mitosis and develop into osteoblasts- found on inner surface of periosteum and endosteum.

Cells of Bone Tissue

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Osteoblasts:

- bone forming cells- found on surface of bone (arrow)- no ability to mitotically divide- collagen secretors

Osteocytes: - mature bone cells- derived form osteoblasts- do not secrete matrix material- cellular duties include exchange of nutrients and waste with blood.

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Osteoclasts

- bone resorbing cells- bone surface- growth, maintenance and bone repair

Abundant inorganic mineral salts:Tricalcium phosphate in crystalline form called hydroxyapatite Ca3(PO4)2(OH)2Calcium Carbonate: CaCO3Magnesium Hydroxide: Mg(OH)2Fluoride and Sulfate

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SKELETAL CARTILAGE

Chondrocytes: cartilage producing cells. Lacunae: small cavities where the

chondrocytes are encased. Extracellular matrix: jellylike ground

substance. Perichondrium: layer of dense irregular

connective tissue that surrounds the cartilage. No blood vessels or nerves

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Types of CartilageHyaline cartilage (glassy) Most abundant cartilageProvides support through flexibilityArticular cartilages and costal cartilage, larynx, trachea, and noseElastic cartilage contains many elastic fibersAble to tolerate repeated bendingEar and epiglottisFibrocartilage resists strong compression and strong tensionAn intermediate between hyaline and elastic cartilageIntervertebral discs and pubic symphysis

BONE FORMATIONThe process of bone formation is called ossificationBone formation occurs in four situations:1) Formation of bone in an embryo2) Growth of bones until adulthood3) Remodeling of bone4) Repair of fractures

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Bone FormationFormation of Bone in an Embryocartilage formation and ossification occurs during the sixth week of embryonic developmenttwo patternsIntramembranous ossificationFlat bones of the skull and mandible are formed in this waySoft spots that help the fetal skull pass through the birth canal later become ossified forming the skullEndochondral ossificationThe replacement of cartilage by boneMost bones of the body are formed in this way including long bones

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Intramembranous OssificationAn ossification center appears in the fibrous connective tissue membrane Osteoblasts secrete bone matrix within the fibrous membraneOsteoblasts mature into osteocytes

Enlargingchondrocytes withincalcifying matrixChondrocytes at the center of the growing cartilage model enlarge and then die as the matrix calicifies.Newly derived osteoblasts cover the shaft of the cartilage in a thin layer of bone.Blood vessels penetrate the cartilage. New osteoblasts form a primary ossification center.The bone of the shaft thickens, and the cartilage near each epiphysis is replaced by shafts of bone.Blood vessels invade the epiphyses and osteo-blasts form secondary centers of ossification.CartilagemodelBoneformationEpiphysisDiaphysisMarrowcavityPrimaryossificationcenterBloodvesselMarrowcavityBloodvesselSecondaryossificationcenterEpiphysealcartilageArticularcartilageReplacement of hyaline cartilage with boneMost bones are formed this way (i.e. long bones).Endochondral Ossification

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Longitudinal Bone GrowthLongitudinal Growth (interstitial) cartilage continually grows and is replaced by bone Bones lengthen entirely by growth of the epiphyseal platesCartilage is replaced with bone as quickly as it growsEpiphyseal plate maintains constant thickness

GROWTH IN LENGTHThe growth in length of long bones involves two major events:Growth of cartilage on the epiphyseal plateReplacement of cartilage by bone tissue in the epiphyseal plate

- osteoblasts located beneath the periosteum secrete bone matrix and build bone on the surface - osteoclasts located in the endosteum resorbs (breakdown) bone.

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GROWTH OF CARTILAGE ON THE EPIPHYSEAL PLATE

- epiphyseal plate (bone length)- 4 zones of bone growth under hGH.1- Zone of resting cartilage (quiescent):- no bone growth- located near the epiphyseal plate- scattered chondrocytes- anchors plate to bone2- Zone of proliferating cartilage (prolferation zone) - chondrocytes stacked like coins- chondrocytes divide

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3- Zone of hypertrophic (maturing) cartilage- large chondrocytes arranged in columns- lengthwise expansion of epiphyseal plate

4- Zone of calcified cartilage- few cell layers thick- occupied by osteoblasts and osteoclasts and capillaries from the diaphysis- cells lay down bone- dead chondrocytes surrounded by a calcified matrix. Matrix resembles long spicules of calcified cartilage. Spicules are partly eroded by osteoclasts and then covered in bone matrix from osteoblasts: spongy bone is formed.

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APPOSITIONAL BONE GROWTHGrowing bones widen as they lengthenAppositional growth growth of a bone by addition of bone tissue to its surfaceBone is resorbed at endosteal surface and added at periosteal surface Osteoblasts add bone tissue to the external surface of the diaphysisOsteoclasts remove bone from the internal surface of the diaphysis

Figure 6-6

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BONE REMODELING

- bone continually renews itself- never metabolically at rest- enables Ca to be pulled from bone when blood levels are low- osteoclasts are responsible for matrix destruction- produce lysosomal enzymes and acids- spongy bone replaced every 3-4 years- compact bone every 10 years

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Fractures: Any bone break.

- blood clot will form around break- fracture hematoma- inflammatory process begins- blood capillaries grow into clot- phagocytes and osteoclasts remove damaged tissue- procallus forms and is invaded by osteoprogenitor cells and fibroblasts- collagen and fibrocartilage turns procallus to fibrocartilagenous (soft) callus

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broken ends of bone are bridged by callusosteoprogenitor cells are replaced by osteoblasts and form spongy bone bony (hard) callus is formedcallus is resorbed by osteoclasts and compact bone replaces spongy bone.Remodeling : the shaft is reconstructed to resemble original unbroken bone. Closed reduction - bone ends coaxed back into place by manipulation Open reduction - surgery, bone ends secured together with pins or wires

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